Sheer laziness?
Tell me how a LCC crew (like Virgin, Jetstar or Tiger) on a 30 minute turnaround (of which only a few short minutes are without deplaning or boarding pax) are meant to open 180 air vents and close all window shades on one side as well as satisfy the company priorities of clear the rubbish, empty rubbish bins, rearrange the seat pockets, vac, stock toilets etc...
In any case, I would suggest having half the window shades closed during boarding prevents the cabin crew of being aware of happenings outside the aircraft during boarding. CASA might have something to say about that.
I would suggest laziness is not your answer. If you read the link to the news article Virgin actually reported the air conditioning system was inop, and a valve required replacing.